Transparent thermoplastic polymers are widely used in place of glass in the manufacture of a variety of products because of their light weight, their resemblance to glass, their economical use, and their excellent impact resistance and other physical properties. For example, transparent blow-molded containers, such as vials, cosmetic bottles, liquid flavoring containers and beverage bottles made of polyethylene terephthalate (PET), are increasingly in demand because they are easily molded and relatively inexpensive. Transparent thermoplastic polymers are used for a variety of other molded, extruded or formed products, such as drinking cups, cooking and eating utensils, food containers, refrigerator storage containers, medical and pharmaceutical tubing and extruded parts, packaging films, extruded sheets and toys. In addition to PET, the transparent polymers commonly used in the manufacture of these products include, for example, styrene acrylonitrile copolymers (SAN), polycarbonates, acrylics, ionomers, polystyrenes, and the like.
Although most transparent plastics have excellent clarity and resemblance to glass, it is desirable in many cases to improve the aesthetic appearance of transparent plastic products by making them translucent, i.e. xe2x80x9cfrostedxe2x80x9d. In the context of the invention, the terms xe2x80x9ctranslucentxe2x80x9d and xe2x80x9cfrostedxe2x80x9d are intended to encompass all gradations of translucency, from almost transparent to almost opaque. Such treatment is intended to impart a softer, more elegant and high grade visual textured appearance to the plastic. Containers and other products having a frosted glass appearance are in demand particularly in the packaging of cosmetics, health, beauty and personal hygiene goods, foods and beverages, as well as for household products, such as frozen food trays, tableware (dishes, cups, plates) and other decorative and utilitarian housewares, and other products, such as cigarette lighters. The frosted glass appearance can be visual only (a smooth surface) or can be both visual and tactile (a rough surface).
Conventional methods for making transparent plastic products translucent include spraying the smooth outer surface of the plastic product with a coating to form a matte surface layer. The plastic article appears frosted because the rough surface diffusely reflects light. Such matte coatings, however, tend to be easily separated from or scratched off the smooth polymer surface due to friction with other articles, and they require an extra production step which adds to production costs. Another method employs a mold with an inner surface that has been roughened to impart a rough or textured matte finish to the molded product. However, the roughened mold may be more expensive to manufacture than a standard mold and, because it is permanent, the mold is limited to producing finished products having a matte surface. Moreover, rough matte surfaces that are designed to resemble ground or frosted glass have tiny projections and recesses that reflect light but may be unpleasant to the touch and easily soiled due to dust and oils transferred from the hands. Oils, in particular, fill recesses and add shine to these rough surfaces. Thus, the amount of diffused light is decreased and any original lustrous appearance may be readily lost.
Other reported methods for achieving a frosted effect in blow-molded bottles and other containers include injection molding of a preformed piece, followed by sandblasting and then heat-crystallizing of the outer surface to roughen and opacify the surface layer prior to the blow molding step. A ground glass effect has been produced in plastic containers by using a blend of olefin resins in which one resin comprises a continuous phase in which another resin is dispersed. Frosted surfaces on bottles have been achieved by heat-crystallizing the outer surface while leaving the inner surface transparent, to form a milky white or translucent effect. Chemical flatting agents have also been employed to opacify acrylic.
Although the above methods produce various types of frosted glass effects, there is still a need for simple and inexpensive methods and compositions for imparting a translucent frosted glass effect to transparent thermoplastic polymers.
The invention provides inexpensive compositions and one-step and two-step methods for imparting a variety of individual translucent optical effects to transparent thermoplastic polymers. The methods and compositions of the invention are particularly useful for imparting a lustrous and rich translucent optical effect to packaging products, such as blow-molded and injection molded products manufactured from polyethylene terephthalate (PET) and styrene acrylonitrile copolymer (SAN), which have traditionally been left transparent because of their excellent clarity and resemblance to glass. Although the invention is herein described with respect to transparent polymers, a translucent effect may also be obtained by the compositions and methods of the invention in polymers that are xe2x80x9cnear transparentxe2x80x9d, such as high density polyethylene and polypropylene. The term xe2x80x9ctransparentxe2x80x9d, as used herein below, is intended to encompass all grades of thermoplastic polymers that are xe2x80x9cnear transparentxe2x80x9d as well as transparent.
The desired translucent optical effect is selectable from a continuum of visual effects from very smooth to very grainy, and is accomplished by selecting an appropriate invention composition and method. Transparent or semitransparent color compounds, pigments and dyes may also be added to the invention compositions to produce colored translucent products.
The invention employs conventional molding, extruding and forming techniques with existing tooling. Thus, the methods do not require expensive extra production steps; nor do they require specialized tools. By the method, a translucent visual frosted glass effect can be imparted to virtually any transparent or near transparent thermoplastic polymer used in the production of molded, extruded or formed products, including films.
In one embodiment of the invention, a one-step method comprises forming a composition comprising (i) 0.01 to 15 parts by weight of at least one particulate, light-diffusing material having an average maximum particle size of about 0.1 micron to about 200 microns, preferably about 1 micron to about 100 microns, and (ii) 85 to 99.99 parts by weight of at least one transparent thermoplastic polymer. The mixture may be molded, extruded or formed by conventional means to form a translucent polymer product.
The particulate material may be in the form of, for example, powders, flakes, platelets, fibers, whiskers, aggregates, agglomerates and mixtures of these. Preferably, the particulate material is selected from the group consisting essentially of calcium sulfates, talc, silicates, kaolin, silicas, mica flakes, mica platelets, mica pearls, titanates, metal sulfates, metal carbonates, sulfides, metal oxides, borides, wollastonite, basalt, boron, boron nitrides, ceramics, naturally occurring calcium carbonates, and mixtures of the foregoing. If the particulate material is boron nitride, it is preferably in the form of for example, powders, aggregates, agglomerates, and the like, or mixtures of these.
In another embodiment of the invention, a two-step method is employed that comprises the steps of forming a concentrate composition which comprises a mixture of (i) 40 to 90 parts by weight of a carrier agent selected from the group consisting of a first transparent thermoplastic polymer, a dispersing agent, and mixtures of these, and (ii) 10 to 60 parts by weight of at least one particulate, light-diffusing material having an average maximum particle size of about 0.1 to 200 microns, preferably about 1 to about 100 microns, as described above. If a mixture of the first polymer and the dispersing agent is employed, the mixture preferably comprises 80 to 98 parts by weight of the first polymer and 2 to 20 parts by weight of the dispersing agent. The carrier agent is preferably finely ground, finely flaked, finely pelletized or a mixture of these, and, more preferably is finely ground. For purposes of this invention, finely ground means a size of about 10 mesh or finer, preferably about 20 mesh; finely flaked means a maximum dimension of about xc2xd inch or less, preferably a maximum dimension of about xc2xc inch or less; and finely pelletized means a maximum diameter of about xe2x85x9 inch or less, preferably a maximum diameter of about {fraction (1/16)} inch or less. The concentrate, in an amount of 0.1 to 15 parts by weight, is then mixed with 85 to 99.9 parts by weight of a second transparent thermoplastic polymer that is chemically compatible with the carrier agent, to form a second mixture which is then molded, extruded or formed by conventional means to form the translucent polymer product. The first and second transparent thermoplastic polymers may be the same or different and are selected from the group described above. The method may optionally include a further step in which the concentrate is extruded and pelletized before adding it to the second polymer.
By either the one-step method or the two-step method, the resulting translucent polymer product comprises 0.01 to 15 parts by weight of the total particulate material and exhibits an average translucency having a measured transmittance that is about 2% to about 65% lower than the transmittance of a polymer part comprising the polymer alone; a haze that is about 900% to about 11,400% higher than the haze of a polymer part comprising the polymer alone; and a clarity that is about 7% to about 95% lower than the clarity of a polymer part comprising the polymer alone, all at a molded part thickness of 30 mils.
A translucent frosted glass effect in transparent thermoplastic molded, extruded or formed polymer products is obtained by the methods and compositions of the invention. The compositions and methods may be employed to impart a translucent optical effect to virtually any transparent or near transparent grade of thermoplastic polymer including, but not limited to, polyolefins (including, but not limited to polypropylene, polyethylene and clarified grades thereof), polyethylene terephthalate, glycol-modified polyethylene terephthalate, glycol-modified polycyclohexanemethanol terephthalate, acid-modified polycyclohexanemethanol terephthalate, polystyrene, styrene acrylonitrile copolymers, polystyrenebutadiene, polystyreneacrylic ester, acrylonitrile-butadiene-styrene, acrylonitrile-styrene-acrylic ester, acrylics, polymethacrylonitrile, polyethylenemethylacrylate, polymethylmethacrylate, polyethylene-ethylacrylate, polyethylenebutylacrylate, polyethyleneacrylic ester, cellulose butyrate, polymethylpentene, polyisobutene, polybutene, polyamides, polycarbonate, ionomers, polyurethane, liquid crystal polymers, cellulose propionate, polyvinyl alcohol, polyethylenevinylalcohol, polyethylenevinylacetate, polyvinyl chloride, high density polyethylene, polypropylene, polyacetal, and copolymers, grafts and blends of the foregoing.
The frosted glass effect may be a visual effect only, such as that obtained when a composition of the invention is extruded, formed, or produced in a mold having a smooth surface, to produce a smooth-surfaced translucent product. Alternatively, the effect may be both visual and tactile, such as that obtained by molding the composition of the invention in a mold having a textured surface to impart a matte finish to the translucent product. As described further below, transparent or semitransparent color concentrates, pigments or dyes may also be blended with the invention compositions to produce colored translucent products, such as a xe2x80x9cpink frostxe2x80x9d, a xe2x80x9cgreen frostxe2x80x9d, a xe2x80x9clavender frostxe2x80x9d etc., in addition to a xe2x80x9cclearxe2x80x9d or xe2x80x9cnaturalxe2x80x9d frosted product. Suitable organic pigments, inorganic pigments and polymer-compatible dyes are known to those skilled in the art of making colored polymers.
The translucent optical effects imparted by the compositions and methods of the invention are achieved by mixing very small quantities of light-diffusing particles, having an average maximum particle size of about 0.1 to about 200 microns, preferably about 1 to about 100 microns, with a transparent thermoplastic polymer prior to molding or extruding the mixture. Preferably, the particles are selected on the basis of their ability to reflect and transmit light diffusely, rather than rectilinearly or specularly, and the translucent visual effect more closely resembles a matte finished molded or spray-coated product. Thus, for example, light-diffusing materials, such as non-shiny mica particles used for laser marking, are preferred over light reflecting (specular) materials, such as mica pearls. However, mica pearls may also be employed to achieve a frosted effect with a more xe2x80x9csatinxe2x80x9d appearance.
To achieve the desired frosted effect, the light-diffusing particles may be in any form, such as powders, fibers, whiskers, platelets, flakes, aggregates, agglomerates or mixtures of these. Suitable particles include, but are not limited to, naturally occurring calcium carbonates, including reagent-grade calcium carbonate, ground chalk, ground limestone, ground marble and ground dolomite; ground or fiber calcium sulfates; silicates, such as glass fibers, glass flakes, solid and hollow glass spheres, aluminum silicate, synthetic calcium silicate and zirconium silicate; talc; kaolin; mica flakes, platelets and pearls; natural silicas, such as sand, quartz, quartzite, perlite, tripoli and diatomaceous earth; fumed silicas; titanates, such as barium titanate; sulfates, such as barium sulfate; sulfides, such as zinc sulfide and molybdenum sulfide; metallic oxides, such as aluminum oxide, zinc oxide, beryllium oxide, magnesium oxide, zirconium oxide, antimony oxide, titanium dioxide and aluminum hydroxide; aluminum diboride flakes; inorganic fibers, such as wollastonite, basalt, boron, boron nitrides and ceramic; single crystal fibers (i.e. whiskers), such as those of alumina trihydrate; short fibers, such as those of aluminum silicate with aluminum and magnesium oxides and calcium sulfate hemihydrate; organic flatting agents, such as wood flour and starch; and mixtures of any of the foregoing. If the particulate material is boron nitride, it is preferably in the form of, for example, powders, aggregates, agglomerates, and the like, or mixtures of these.
A desired translucent optical effect ranging in a continuum from very smooth visual textured effects to very grainy visual textured effects may be achieved, depending on the particulate material or mixture of particulate materials selected and the quantity of the particulate employed. For example, a smooth visual translucency is obtained by using white powder particulates, such as barium sulfate, zinc sulfide or ultrafine ground chalk. Slightly grainy visual translucency is obtained by using transparent particulates, such as solid glass microspheres having a particle diameter of about 2 to about 100 microns (preferably about 4 to about 44 microns) or hollow glass microspheres having a particle diameter of about 10 to 100 microns (preferably about 65 to about 75 microns); whereas a slightly more grainy visual translucency is obtained by using ceramic fibers having a diameter of about 2 to about 12 microns, and lengths of about 45 microns to about 1.5 millimeters (mm). Grainy translucent visual effects are also obtained with additives such as lamellar kaolin having an aspect ratio of 10:1 (length:diameter). To obtain very grainy visual translucent effects, wollastonite having aspect ratios ranging from about 5:1 to 15:1, are employed, with the highest aspect ratios giving the grainiest effects. Very grainy translucent visual effects are also achieved by using whiskers, such as such as those of alumina trihydrate, and metal flakes or platelets, such as those of mica.
Exemplary suitable particles for use in the invention are Sachtleben Blanc Fixe Micro(copyright) 2278N (milled barium sulfate, approximately 3 microns, available from Whittaker, Clark and Daniels, Inc., South Plainfield, N.J. (manufacturer Sachtleben, Germany); Omyacarb(copyright) 4 (calcium carbonate, 3.5 micron median, 15 micron max, Omya Inc.); Talc 399 (talc (magnesium silicate), available from Whittaker, Clark and Daniels, Inc., South Plainfield, N.J. (manufacturer Specialty Mineral); Zeeospheres(copyright) W-610 (ceramic microspheres, mixture of particle sizes of approximately 2 to 45 microns, Zeelan Industries, St. Paul, Minn.); Silcron(copyright) G602 (fine particle silica, average particle size approximately 2.7 microns, SCM Pigments, Baltimore, Md.); NYAD G(copyright) Wollastocoate (wollastonite, aspect ratio 15:1, 100-325 mesh), NYAD(copyright) 400 wollastonite (aspect ratio 5:1), 400 Wollastocoate (aspect ratio 5:1, 400 mesh) (NYCO Minerals, Inc., Willsboro, N.Y.); hollow glass microspheres (glass bubbles, 3M Corporation); Acematt(copyright) TS 100 (silica flatting agent, average particle size approximately 2 to 10 microns, Degussa Corp., Ridgefield Park, N.J.); Iriodin(copyright)/Lazer Flair(copyright) LS 810 (mica-based additive, particle size approximately 2 to 28 microns, EM Industries, Hawthorne, N.Y.); Afflair(copyright) 110 Fine Satin (mica-based additive, E.M. Industries, Hawthorne, N.Y.); Polartherm(copyright) (PT110 (Advanced Ceramics Corporation, Cleveland, Ohio; boron nitride particulate material, exhibiting a particle size distribution as follows: 10% of particles 23.770 microns or smaller, 50% of particles 49.920 microns or smaller, and 90% of particles 73.710 microns or smaller); and Carborundum Carbotherm(copyright) AS0517 (Carborundum Corporation, Amherst, N.Y.; boron nitride particulate material, agglomerates: approximate particle size 30 microns).
Because the quantities of the particulates employed in the invention compositions and methods are extremely small, the particulates do not perform the traditional functions of fillers (e.g. reinforcers, extenders, opacifiers, plasticizers, etc.).
In one embodiment of the invention, a one-step method for imparting a translucent optical effect to a transparent thermoplastic polymer is employed, and comprises the steps of forming a substantially homogeneous composition comprising a mixture of (i) 0.01 to 15 parts by weight of at least one particulate, light-diffusing material having an average maximum particle size of from about 0.1 microns to about 200 microns, and (ii) 85 to 99.99 parts by weight of a transparent thermoplastic polymer; and molding, extruding or forming the homogeneous mixture to form a translucent molded, extruded or formed polymer product. Preferably the particles have an average maximum size of from about 1 to about 100 microns. Preferably, the mixture comprises 0.1 to 6 parts by weight of the particulate material, more preferably, 0.2 to 5 parts by weight, even more preferably, 0.5 to 2 parts by weight, and most preferably 0.5 to 1.5 parts by weight of the particulate material.
In this embodiment of the invention, to achieve a substantially homogeneous mixture of the particulates and the polymer for a homogeneous translucent optical effect, it is preferred that the polymer be finely ground 20-mesh powder. The pelletized polymer may be finely ground to a 20-mesh powder prior to mixing with the particulates or the polymer may be purchased as a finely ground powder, when available. As discussed further below, a dispersing agent and/or a flow enhancing (anti-bridging) agent may also be added to the particulate mixture to aid in achieving homogeneity. For practical purposes, when mixing large amounts of polymer with particulates, the polymer will not be pre-ground in powder from but may be used in commercially available pellet form (average diameter {fraction (1/16)} inch to xe2x85x9 inch or greater). The achievable homogeneity of a pelleted polymer/particulate mixture, however, depends upon such factors as the type of particulate employed, the pellet and particulate diameter or size, the mixing time, the natural segregation of the components during the time period before use, and the like, resulting in a product which may have a variable, rather than a homogeneous, overall translucent appearance. Thus, this embodiment of the composition and method is less preferred if a high degree of homogeneity of the optical effect is desired. Homogeneity of a pelleted polymer/particulate mixture may be improved by separately metering the polymer pellets and the particulates (frosting agents, and/or dispersants and/or flow enhancers) through separate feed lines into the melting screw portion of any device used during the melt mixing phase of the extruding, molding or forming process.
In another embodiment of the invention, a two-step method is employed. By this method, a substantially homogeneous concentrate mixture comprising at least one particulate material in a carrier agent is prepared. A desired quantity of this concentrate is then blended with a chemically compatible polymer (let down resin) to form a second mixture, which is then molded, extruded or formed and cured, as described above, to form the translucent polymer product. The degree of translucency can be adjusted by increasing or decreasing the loading (i.e. the xe2x80x9clet down ratioxe2x80x9d of concentrate to let down resin) of the concentrate in the end product.
The two-step method comprises the steps of forming a concentrate composition comprising a mixture of (i) 40 to 90 parts by weight of a carrier agent that is finely ground, finely flaked, finely pelletized, or a mixture of these, selected from the group consisting essentially of a first transparent thermoplastic polymer, a dispersing agent, or mixtures of these, and (ii) 10 to 60 parts by weight of at least one particulate, light-diffusing material as described above, to form a second mixture that comprises 0.01 to 15 parts by weight of the composition and 85 to 99.99 parts by weight of a second transparent thermoplastic polymer that is chemically compatible with the carrier agent. The second and first transparent thermoplastic polymers may be the same or different and are selected from the group described above. For purposes of this invention, finely ground means a size of about 10 mesh or finer, preferably about 20 mesh; finely flaked means a maximum dimension of about xc2xd inch or less, preferably a maximum dimension of about xc2xc inch or less; and finely pelletized means a maximum diameter of about {fraction (1/16)} inch or less, preferably a maximum diameter of about xe2x85x9 inch or less. The dispersing agent comprises a low molecular weight substantially transparent polymeric material, such as a silicone wax, a fatty acid, a metallic salt, an ionomer wax, an amide wax, a hydroxy stearate, an olefinic wax, or a mixture of any of the foregoing, and is preferably a bis-stearamide or a hydroxy stearate.
The concentrate and second polymer may be combined in a process during the melt mixing phase, such as by their separate metering into the melting screw portion of the device through separate feed lines. Alternatively, the concentrate and the second polymer may be mechanically combined prior to the introduction of the mixture into a molder or extruder. The mixture is then molded or extruded to form a translucent polymer product.
The carrier agent may comprise any agent that is capable of forming a substantially homogeneous dispersion therein of the particulate material. The carrier agent may comprise finely ground (e.g. 20 mesh) polymer pellets, a polymer that is commercially available finely ground, a combination of powdered and pelletized polymers, or a finely ground or finely flaked dispersing agent, such as a silicone wax, fatty acid, metallic salt, ionomer wax, amide wax, hydroxy stearate, olefinic wax, or a mixture of any of these. Exemplary dispersing agents comprise a bis-stearamide, such as ethylene-bis-stearamide, or a hydroxy stearate, such as Castorwax(copyright) (Caschem, Bayonne, N.J.). To prepare finely ground polymer pellets, commercially available polymer pellets are ground by conventional methods, such as in an ambient or cryogenic grinder, to about a 20-mesh powder. Some polymers are also commercially available finely ground. Fine flaking of the wax-base dispersing agent is achieved by known methods to form flakes that are typically irregular or uneven and preferably have a maximum dimension of xc2xc inch.
Optionally, when the finely ground polymer is employed as the carrier agent, 0 to 20 parts per weight of a dispersing agent and/or 0.1 to 7 parts, preferably 1 to 2 parts, of a flow enhancing or anti-bridging agent, such as fumed or precipitated silica, may be added to the mixture. One to 2 parts by weight of a flow enhancing agent may also be added to the mixture when a dispersing agent is employed as the carrier agent. Other additives, known to those skilled in the art of polymer compounding, may include anti-oxidants, UV absorbers/light stabilizers, and the like, in quantities that do not contribute substantially to or interfere with the translucent optical effect. In addition, transparent or semi-transparent colorants, pigments and dyes may be added to the mixture to provide colored translucent products.
For purposes of this invention, a translucent optical effect is defined by measurements of the resulting polymer product (after hardening), of the transmittance, haze, and clarity. Translucence is defined as a transmittance that is about 2% to about 65% lower than the transmittance of a polymer part comprising the polymer alone; a haze that is about 900% to about 11,400% higher than the haze of a polymer part comprising the polymer alone; and a clarity that is about 7% to about 95% lower than the clarity of a polymer part comprising the polymer alone, all at a molded part thickness of 30 mils. Measurements were made according to the procedures described in the operating manual of a HazeGard(copyright) Plus hazemeter (Byk-Gardner), which made reference to ASTM D-1003 and D-1044. The transmittance is a measurement of the amount of light transmitted through a sample, compared to the amount of light incident upon the sample (in a perpendicular beam). Haze is a measurement of the transmitted light scattered more than 2.5xc2x0 from the axis of the incident light. Clarity is a measurement of the amount of the transmitted light scattered less than 2.5xc2x0 from the axis of the incident light.
A translucent effect for purposes of this invention may also be defined by a measurement of the translucency of the polymer product, after hardening, by its contrast ratio. The contrast ratio is the ratio of the percent reflectance of a sample over a white background and the percent reflectance of the sample over a black background. Contrast ratios from 0 to 100 are obtainable, with samples having ratios of greater than 97 being considered opaque. When different samples are measured over the same white and black backgrounds, contrast ratios may be used for comparison of the relative degrees of opacity between samples. The polymer product formed by either the one-step method or the two-step method described above comprises 0.01 to 15 parts by weight of the particulate material and exhibits, after hardening, an average translucency having a contrast ratio that is about 2% to about 60% higher than the contrast ratio of a polymer part comprising the polymer alone, at a molded part thickness of 0.030 inches (30 mils).